1. The Field of the Invention
Embodiments of the present invention relate to design and manufacture of novel magneto-optical display elements. These magneto-optical elements have numerous uses in a variety of applications including sensor elements, games, etc but most notably information displays and large-format, reflective, low-cost bistable displays. These resulting displays are well suited for a variety of applications including indoor and outdoor message boards, scoreboards, clocks, temperature displays, variable message signs, transportation information displays and the like.
2. The Relevant Technology
While magnetics itself and the study of magnetic materials is an old discipline that dates back centuries and is still studied heavily today (see Spaldin, N., Magnetic Materials Fundamentals and Device Applications, Cambridge Univ. Press, 2003; Kittel, C. Introduction to Solid State Physics, Wiley, 1996), few attempts have been made to develop magnetic-optical materials suitable for displays. The study of magnetic materials themselves has resulted in significant technological advances, such as in 1898 and 1928 by Poulsen and Pfleumer, respectively, leveraging magnetic phenomena in the development of magnetic in recording media. Further, in the 1960's magnetic materials were used to store memory, first by Forrester et al and others (U.S. Pat. Nos. 2,736,880, 2,667,542, 2,708,722). Magnetic core memory, pioneered by Olsen and others (U.S. Pat. Nos. 3,161,861, 4,161,037, 4,464,752) has seen significant attention for decades due to its implications on the computing industry but for a number of reasons was replaced by silicon-based memory devices.
Starting in the early 1900s, there were various electronic signage technologies that used magnetics in the form of electromagnetic actuators (Naylor U.S. Pat. No. 1,191,023, Taylor U.S. Pat. No. 3,140,553 and Browne U.S. Pat. No. 4,577,427). These discrete actuators were generally variations of small electromagnetic coils or motors with a reflective mechanical flap apparatus. They were broadly used and a small number of these devices are still in use today for specific outdoor signage applications like score boards and transit signs. However, they have become obsolete with the advent of solid-state light emitting diodes (LEDs). In addition, these devices were generally expensive to construct, suffered from limited reliability and resolution due to the size of the discrete mechanical assemblies needed for each display pixel.
Development continued in the use of magnetic actuators for displays by Weiacht (U.S. Pat. No. 6,510,632) where magnets were attached to large flaps or “flags” that could be rotated 180 degrees in forming a signage character. This type of technology was continued with the patents of Fischer et al and others (U.S. Pat. Nos. 6,603,458, 3,936,818) again discussing the use of magnetically driven flaps as individual display pixels. These devices are mounted on a mechanical axis and only one magneto-optic “flag” is used in each individual device.
More recently, some work has been accomplished with respect to magnetic-optical materials suitable for display technologies. Here, workers utilized bi-colored magnetic particles instead of mechanical flaps. The use of smaller discrete magnetic spheres has been envisioned as a display design by Magnavox (Lee, L. IEEE Transactions on Electron Devices, ED-22, P758) and more recently by Katsuragawa et al. (Japan Patent Publication 2002-006346) and to a lesser extent by Masatori (Japan Patent Publication 08-197891). This art deals primarily with the use of small magnetic particles which respond to an external magnetic field by rotating. The rotation of these magnetic particles is in response to the external field and the particles desire to lower their potential energy. Here, as shown in FIG. 1, the magnetic elements are spherical in shape and are magnetized such that the magnetic dipoles, shown at 1, are perpendicular to their plane of color separation, shown at 2. Moreover, these magnetic particles are extremely small, in the micron-sized range, having magnetic and optical orientations significantly different from those disclosed in herein. These micron-sized elements would require very technically advanced and expensive processes in their fabrication such as gas-phase formation and/or coating, colloid-growth processes or the like.
The vast majority of work performed with respect to magnetics exhibiting optical effects revolves around phenomena like the Kerr Effect (J. Kerr, Phil. Mag. 3, 321, 1877) which relates to changes of light reflection off magnetic materials. This phenomenon is in large part, the basis for the field of optical isolators, and very advanced materials showing such effects such as CdxMnyHgzTe (Onodera, U.S. Pat. No. 5,596,447).
Permanent magnetic materials (O'Handley, R. C. Modern Magnetic Materials Principles and Applications, Wiley, 2002), by which magneto-optical elements can be formed are generally limited to permanent materials such as Ferrites (with the formula AB2O4, spinel structure, e.g. ZnFeO4), alloys such as “Alnico” (Aluminum, Nickel and Cobalt alloys, often with added Iron, Copper or Titanium), Permalloy (a Nickel Iron Alloy) or magnetically stronger rare-earth magnets, most notably Neodymium Iron Boron (“Neo,” like Nd2Fe14B) or Samarium Cobalt (alloys similar to SmCo5). These rare-earth magnetic materials have vastly superior magnetic strengths compared to other magnetic materials and higher curie temperatures that increase their effective operating temperature range. Rare-earth magnets typically have static magnetic field strengths as high and even greater then 1.2 Tesla, very high compared to their non-rare-earth counterparts which typically have field strengths as low as 50-100 milliTesla.
The invention disclosed herein uses significantly different architectures, materials selections and fabrication methods compared to any previous magnetic-optical materials, compositions, composites or materials used in display fabrication.